This invention pertains to a process of recovering propylene oxide in purified form from an epoxidation reaction product, preferably, a reaction product obtained from the epoxidation of propylene with hydrogen peroxide in the presence of a titanium-containing zeolite catalyst.
Propylene oxide finds utility as a starting material in the preparation of polypropylene polyether polyols, which find utility in the manufacture of polyurethane polymers. Propylene oxide used for such purposes is required to meet strict purity specifications, so as to avoid disadvantageous effects in downstream polyurethane products.
Epoxidation processes comprising the reaction of propylene with hydrogen peroxide in the presence of titanium-containing zeolite catalysts are known in the art, as illustrated, for example, in international patent publication WO-A1-02/14298 (Enichem S. P. A.) Such processes are typically conducted in the presence of a reaction solvent, preferably, methanol, which tends to promote high catalyst activity and selectivity. Such processes produce water as a co-product and minor amounts of glycols, glycol ethers, acetaldehyde, acetone, and propionaldehyde as by-products. Accordingly, a crude propylene oxide product stream obtained from such epoxidation processes contains in addition to the propylene oxide substantial quantities of reaction solvent, typically methanol, and water, as well as impurity amounts of glycols, acetaldehyde, acetone, and propionaldehyde.
The purification of the crude propylene oxide product is known to be difficult. It is especially difficult to remove methanol and acetaldehyde to a high degree of efficiency. Nevertheless, commercial grade propylene oxide requires that the product contains no greater than 10 parts per million (ppm) methanol; no greater than 100 ppm water; and no greater than 30 ppm aldehydes.
The purification of propylene oxide has been considered in the prior art. Certain disclosures, represented by U.S. Pat. No. 4,140,588, teach the extractive distillation of propylene oxide with water for the purpose of removing contaminating quantities of methanol and acetone. Typically, a crude feed comprising from 92 to 99 percent propylene oxide and small quantities of water, methanol, and acetone is fed to a lower portion of an extractive distillation zone. Water is fed at a higher point on the column, and the extractive distillation is operated at a bottoms temperature ranging from 60° C. to 100° C. An overhead distillate is obtained comprising a substantially pure propylene oxide; while a bottoms fraction is obtained comprising predominantly methanol, water, acetone, and disadvantageous amounts of propylene oxide and propylene glycol. Typically, the disclosed process suffers a yield loss of propylene oxide that is greater than 1 percent and as high as 2.5 percent.
Other art, represented by U.S. Pat. No. 5,849,938 and EP-B1-1,009,746, discloses the separation of propylene oxide, acetaldehyde, and methanol by extractive distillation using water or propylene glycol as extraction solvent. The crude epoxidation mixture, containing from 2 to 10 percent propylene oxide, 50 to 85 percent methanol, 10 to 30 percent water, and 0.01 to 0.1 acetaldehyde by weight, is introduced into an intermediate section of the distillation tower. A bottoms temperature from 90° C. to 120° C. is maintained, such that a bottoms stream is obtained containing methanol, water, any further extractive solvent, and a substantial portion of the acetaldehyde. A purified propylene oxide is obtained; but disadvantageously, the concentration of methanol in the purified propylene oxide remains higher than acceptable for most applications. Moreover, the references do not address the yield loss of propylene oxide in the extractive distillation process due to side-reactions of propylene oxide with the extractive solvent and methanol in the feed.
Other art, represented by EP-A1-1,122,248 and WO-A1-01/57010, discloses the work-up of an epoxidation product stream containing propylene, propylene oxide, methanol, and an organic solvent, such as methanol. The work-up comprises separating the product stream in a pre-evaporator such that between 20 and 60 percent of the amount of organic solvent and more than 95 percent of the propylene oxide fed is removed with the overhead product. The residue of the organic solvent and over 90 percent of the water fed is contained in the bottom product. Thereafter, the propylene remaining in the overhead is stripped in a C3 stripper. The recovered product mixture is thereafter subjected to extractive distillation using a polar solvent, such as water, so as to obtain a purified propylene oxide in the overhead product and methanol and polar solvent in the bottoms product. The references are silent with respect to various aspects of the extractive distillation, the quality of purified propylene oxide obtained, and the yield loss of propylene oxide.
Other art, represented by EP-B1-1003733 and U.S. Pat. No. 6,024,840, discloses the separation of methanol and acetaldehyde from a crude epoxidation reaction product comprised of 2 to 10 percent propylene oxide, 60 to 85 percent methanol, 10 to 25 percent water, 0.01 to 0.1 percent acetaldehyde, and 0.01 to 0.1 propylene, by weight. It is taught to fractionate the crude epoxidation reaction product at a reflux:distillate ratio generally of from 10:1 to 30:1 to obtain a bottoms stream comprising methanol, water, and at least 99 percent of the acetaldehyde, and to obtain an overhead stream comprising propylene oxide, propylene, and residual methanol, but substantially devoid of water and acetaldehyde. It is further disclosed to remove propylene from the overhead stream in a second distillation. Thereafter, the resulting propylene oxide stream devoid of propylene is taught to be subjected to extractive distillation using a polar solvent, such as propylene glycol, generally at a bottoms temperature of from 80° C. to 110° C. From the extractive distillation, a bottoms stream is obtained containing the extractive solvent, methanol, water, and other impurities; while a purified propylene oxide is obtained as overheads. Disadvantageously, the high bottoms temperature of the extractive distillation might cause unacceptable yield loss of propylene oxide due to by-product formation. Moreover, the concentration of methanol illustrated in the purified propylene oxide is too high for most applications.
In consequence of the above, a need exists for improvements in the separation and purification of propylene oxide reaction products, preferably those obtained from the epoxidation of propylene with hydrogen peroxide. An efficient and cost-effective separation scheme that produces commercial grade propylene oxide meeting required purity standards would be highly desirable. Due to the high boiling point of propylene glycol, it would be more desirable if the separation method did not require this component as an extraction solvent. It would be even more desirable if the separation method did not produce unacceptable losses of propylene oxide due to by-product formation with methanol or extractive solvents in the crude product.